Huawei H12-831_V1.0 Exam With Confidence Using Practice Dumps

???? Background:

This is an MPLS VPN with OSPF as the PE-CE routing protocol.

CE2 is dual-homed to both PE2 and PE3, creating the possibility of route feedback/loops between the PEs through CE2.

???? What Causes the Loop?

Normally, when a route is redistributed from BGP into OSPF on the PE, the OSPF DN bit is set in the Type 3 or Type 5 LSA.

The DN bit tells other routers: “Do not redistribute this route back into BGP.”

CE2, not understanding the DN bit, may ignore it and re-advertise the route from PE2 to PE3 and vice versa.

❗ Problem:

If both PE2 and PE3 accept the redistributed LSA from CE2 and re-inject it into the VPN, a routing loop can occur.

???? Solution:

To avoid this, Huawei supports the command:

bash

CopyEdit

dn-bit-set disable

This command disables DN-bit marking on the PE, avoiding potential loop prevention issues with non-DN-bit-aware CEs.

✅ Final Answer: A. TRUE

Official Huawei Extract:

" In dual-homing OSPF VPN scenarios, routing loops may occur due to Type 3 LSA exchange if the CE does not process DN-bit. Running dn-bit-set disable on the PE prevents loop issues by not setting the DN-bit. "

— [Huawei HCIA-Datacom Certification Guide – OSPF in MPLS VPN and DN-Bit Handling]

Comprehensive and Detailed In-Depth Explanation:

To determine the cost value of the route 10.0.35.0/24 in the routing table of R1, let ' s analyze the possible shortest paths from R1 to R3 (which owns the 10.0.35.0/24 network) based on the given OSPF link costs:

Path 1: R1 → R2 → R3

R1 to R2: Cost = 20

R2 to R3: Cost = 10

Total Cost = 20 + 10 = 30

Path 2: R1 → R4 → R3

R1 to R4: Cost = 10

R4 to R3: Cost = 10

Total Cost = 10 + 10 = 20 ✅ (Lowest cost path)

Path 3: R1 → R5 → R3

R1 to R5: Cost = 10

R5 to R3: Cost = 10

Total Cost = 10 + 10 = 20 ✅ (Equal-cost path)

Final Calculation:

The two best equal-cost paths (R1 → R4 → R3 and R1 → R5 → R3) have a cost of 20.

Since OSPF supports equal-cost multi-path (ECMP) routing and maximum load-balancing 8 is enabled, OSPF will install both routes into the routing table with the same cost of 20.

The path R1 → R2 → R3 has a higher cost (30) and will not be chosen.

Final Answer: ✅ 20 (Option C)